Lithium polymer battery charger apparatus

ABSTRACT

Lithium polymer battery charger apparatus for charging a plurality of equal charge point lithium polymer battery cells prevent overcharging of any cell, whether the cells are arranged in a series stack or are arranged in parallel. When the cells are connected in a series stack, a power supply is connected to the series stack to apply a charge current to the series stack. The state of charge of each cell in the stack is monitored. Information that the state of charge of any cell is approaching full charge is used to control the charge current and to prevent overcharging of any cell.

BACKGROUND OF THE INVENTION

This invention relates to lithium polymer battery charger apparatus.

This invention relates particularly to lithium polymer battery chargerapparatus which prevent overcharging of any cell, whether the cells arearranged in a series stack or are arranged in parallel.

Lithium polymer battery cells are unforgiving of overcharge.

An overcharged lead acid battery will electrolyze some easily replacedwater, and nickel-cadmium or metal-hydride batteries have voltages whichstop rising at full charge; but the voltage of the lithium polymerbattery cell continues to rise even while being overcharged.

The voltage of a lithium polymer battery rises quite distinctly at theend of charge.

For a lithium polymer battery having a fully charged voltage of as muchas 4.5 volts, the voltage can rise to a voltage of about 4.7 volts.

What is unique in the lithium polymer battery cell art is that lithiumpolymer battery cells will not stand such overcharging. The overchargingmust be avoided. Overcharging damages the battery cell seriously. Thereis no liquid to vaporize (as there is in a liquid acid battery, forexample). Lithium polymer batteries work on the idea that lithium ionsare embedded in a solid polymer. The lithium polymer battery cell is asolid battery. Overcharging can be visualized as depleting all of thelithium ions off of the plate. Breaking connection by overchargingresults in a condition in which there is no way to re-establish theconnection.

It is essential to avoid overcharging of a lithium polymer battery cell.

SUMMARY OF THE INVENTION

The lithium polymer battery charger apparatus of the present inventionprevent overcharging of any cell, whether the cells are arranged in aseries stack or are arranged in parallel.

In one embodiment of the present invention, a plurality of equal chargepoint lithium polymer battery cells are connected in a series stack. Apower supply is connected to the series stack. The power supply appliesa charge current through the series stack. The state of charge of eachcell in the stack is monitored. The monitoring detects when the state ofcharge in any cell is approaching full charge. The charge current toeach cell is controlled in response to the detecting of the state ofcharge to prevent overcharging of any cell.

In one embodiment of the present invention, the controlling of thecharge current includes discontinuing the charge current to the seriesstack when any one cell reaches full charge.

In another embodiment of the present invention, the controlling of thecharge current includes reducing the charge current to a trickle chargeto the series stack when any one cell reaches full charge.

In several embodiments of the invention, the detecting of informationthat a particular lithium polymer battery cell is nearing full charge isused to shunt sufficient current around that cell to avoid overchargingthat cell and to continue shunting around that cell until all of thecells have been fully charged. The shunting around a particular cell isdone, in some embodiments of the present invention, in an analogtransition in which the amount of shunted current is increased as thecell gets nearer to a fully charged condition. The shunting is done, insome other embodiments of the present invention, in a digital transitionin which a relay is energized to shunt a fixed amount of current eitherwhen the cell reaches a fully charged condition or for a predeterminedperiod of time during the charging cycle before the cell reaches thefully charged condition.

A single power supply may be used for charging a single series stack ofcells or, in accordance with other embodiments of the present invention,multiple power supplies may be used for supplying multiple chargecurrents with a separate, series stack of cells connected to each powersupply.

In one embodiment of the present invention, the multiple power suppliescomprise multiple, equally matched, secondary windings of a transformer.

In some embodiments of the present invention, the charge current isconstant current.

In other embodiments of the present invention, the charge current isvaried during the charging operation.

In some embodiments of the present invention, the charging history ofeach cell is determined and the charge current to the cell is varied independence on the charging history and/or detection of the state ofcharge of the individual cell.

In one specific embodiment of the present invention, a plurality ofequal charge point lithium polymer battery cells are charged by using atransformer having one primary winding and a plurality of equallymatched secondary windings. Each secondary winding is connected to atleast one lithium polymer battery cell. The voltage on the primarywinding is controlled to produce a voltage in each secondary windingwhich is effective to bring each lithium polymer battery cell to a fillycharged condition.

In one specific embodiment of the invention, only one lithium polymerbattery cell is connected to a secondary winding. The maximum voltage onthe primary winding is limited to that which will produce a voltage oneach secondary winding corresponding to the voltage needed to bring thelithium polymer battery cells to the fully charged condition, and noadditional monitoring circuitry is needed to avoid overcharging of thelithium polymer battery cells.

In other embodiments of the invention, a plurality of cells areconnected in series with a particular secondary winding; and, in thatevent, each cell is instrumented to detect when the cell is nearing fullcharge. Sufficient current is then shunted around that cell to avoidovercharging that cell, and the shunting is continued until all of thecells in the series stack have been fully charged. The shunting is donein an analog transition or in a digital transition.

Apparatus which incorporate the features described above and which areeffective to function as described above constitute further, specificobjects of this invention.

Other and further objects of the present invention will be apparent fromthe following description and claims and are illustrated in theaccompanying drawings, which by way of illustration, show preferredembodiments of the present invention and the principles thereof and whatare now considered to be the best modes contemplated for applying theseprinciples. Other embodiments of the invention embodying the same orequivalent principles may be used and structural changes may be made asdesired by those skilled in the art without departing from the presentinvention and the purview of the appended claims.

BRIEF DESCRIPTION OF THE DRAWING VIEWS

FIG. 1 is a graph showing the charge and discharge characteristics of alithium polymer battery with the voltage plotted as the verticalcoordinate and with the percent capacity of charge plotted as thehorizontal coordinate. FIG. 1 shows that a lithium polymer battery cellhaving a fully discharged voltage of 2.0 to 2.5 volts and a fullycharged voltage of 4.0 to 4.5 volts depending upon the negativeelectrode (anode) carbon material form. FIG. 1 also illustrates that thelithium polymer battery cell will continue to charge to a higher voltagethan its fully charged 4.0 to 4.5 volts if charging is continued afterthe lithium polymer battery cell reaches its fully charged 4.0 to 4.5volts. Charging beyond the fully charged condition produces irreparabledamage to the lithium polymer battery cell.

FIG. 2 is a schematic diagram showing a lithium polymer battery chargerconstructed in accordance with one embodiment of the present invention.The lithium polymer battery cells to be charged are indicated by thereference numerals 13, 15, 17, and 19 in FIG. 2.

FIG. 3 is a schematic view showing a plurality of equal charge pointlithium polymer battery cells connected in a series stack. Voltagesurvey means monitor the state of charge of each cell in the stack. FIG.3 also shows a controlled current source connected to the series stack.The controlled current source is associated with the voltage surveymeans so that the amount of current supplied from the controlled currentsource is controlled in response to a control signal supplied from thevoltage survey means.

FIG. 4 is a schematic view showing a plurality of lithium polymerbattery cells connected in a series stack. Voltage monitors monitor thevoltage of each cell in the stack. FIG. 4 also illustrates a relaycontrolled resistor set associated with the cells for shunting currentaround individual cells at selected periods of time during the chargingof the series stack of lithium polymer battery cells.

FIG. 5 is a schematic view like FIG. 4 but showing another embodiment ofthe invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Lithium polymer battery cells are unforgiving of overcharge.

An overcharged lead acid battery will electrolyze some easily replacedwater, and nickel-cadmium or metal-hydride batteries have voltages whichstop rising at full charge; but the voltage of the lithium polymerbattery cell continues to rise even while being overcharged.

FIG. 1 shows how the voltage of a lithium polymer battery rises quitedistinctly at the end of charge.

Thus, as illustrated in FIG. 1, for a lithium polymer battery cellhaving a fully charged voltage of 4.0 to 4.5 volts, the voltage can riseto a voltage of about 4.7 volts. See the overcharge area peak indicatedby the reference numeral 11 in FIG. 1.

What is unique in the lithium polymer battery cell art is that lithiumpolymer battery cells won't stand such overcharging. The overchargingmust be avoided. Overcharging damages the battery seriously. There is noliquid to vaporize (as there is in a lead acid battery, for example).Lithium polymer batteries work on the idea that lithium ions areembedded in a solid polymer. The lithium polymer battery cell is a solidbattery. Overcharging can be visualized as depleting all of the lithiumions off of the plate. Breaking connection by overcharging results in acondition in which there is no way to re-establish the connection.

It is essential to avoid overcharging of a lithium polymer battery cell.

The lithium polymer battery charger methods and apparatus of the presentinvention, as will be described in more detail below with reference toFIGS. 2-4, prevent overcharging of any cell, whether the cells arearranged in a series stack or are arranged in parallel.

FIG. 2 is a schematic diagram illustrating a lithium polymer batterycharger 10 constructed in accordance with one embodiment of the presentinvention.

In FIG. 2 the lithium polymer battery cells to be charged are indicatedby the reference numerals 13, 15, 17, and 19.

Each cell is to be charged from a voltage as low as the 2.0 totallydischarged voltage to the fully charged voltage as high as 4.5 volts.

The power supply for supplying the charged current is a transformer T1having a single primary and having two matched secondary windings. Thefirst secondary winding is associated with diodes D1, D2 and D3. Thesecond secondary winding is associated with diodes D4, D5, and D6.

While only two matched secondary windings have been illustrated in FIG.2, it should be noted that additional matched secondary windings couldbe incorporated in the charger shown in FIG. 2 for charging additionallithium polymer battery cells.

As will also become more apparent from the description to follow, asingle lithium polymer battery cell could be associated with a singlesecondary winding, rather than having two cells connected in series witheach secondary winding as illustrated in FIG. 2; and, in that event,monitoring circuitry for monitoring the charge voltage of the lithiumpolymer battery cells could be eliminated.

By way of further introductory comments with respect to FIG. 2, itshould also be noted that more than two lithium polymer battery cellscould be associated with each secondary winding.

In the specific embodiment 10 shown in FIG. 2 the transformer T1 servesas a constant current power supply.

The output voltage sampler and comparator 23 and the pulse widthmodulator 21 are effective to control operation of the transformer T1 sothat each of the secondary windings functions as a constant currentpower supply for the cells associated with a particular secondarywinding.

In the embodiment 10 shown in FIG. 2, each secondary winding has twocells connected in a series stack. For example, cells 13 and 15 areconnected in a series stack to the lower secondary winding, and cells 17and 19 are connected in a series stack to the upper secondary winding.

In the embodiment 10 shown in FIG. 2, the state of charge of each cellis monitored so that the battery charger 10 detects when the state ofcharge in any cell is approaching full charge.

The charge current to each cell is controlled in response to thedetection of the state of charge of each cell to prevent overcharging ofany cell.

In FIG. 2 the battery charger 10 starts shunting current around a cellwhen the voltage across that cell gets up to a certain point.

Operationally, the battery charger 10 shown in FIG. 2 trickle charges aparticular cell after the voltage across the cell gets up to a selectedpoint, as will now be described in more detail below.

In the battery charger embodiment 10 shown in FIG. 2, the cell 17 isgenerating a voltage V11 with respect to the Grnd2. A voltage regulatorVR11 is putting out a 2.4 volts reference. A resister divider takes thevoltage V11 which is the voltage across the cell 17. This resisterdivider is set up so that its center tap is at 2.4 volts, the output ofthe voltage regulator VR11, when the cell is fully charged. The OpAmpU11 thus has a positive terminal reference which is the 2.4 voltsexactly. The voltage at the negative terminal of the OpAmp U11,representing the output of the resister divider, slowly rises up as thecell 17 takes the charge. When the cell voltage gets high enough, theoutput of the OpAmp U11 goes in the negative direction. This turns onthe power transistor Q11 to shunt current from V11 around that cell 17and back to Grnd2. So, as the voltage across the cell 17 gets up to acertain point, the transistor Q11 starts shunting charge current aside.This operationally reduces the charge current in the cell 17. Thebattery charger 10 thereafter holds the cell 17 at constant voltage, andthe charge current that the cell 17 does not absorb goes through theshunting device Q11. The capacitor C11 is useful to limit the frequencyresponse of the electronics so that the electronics do not oscillate.

In this instance the information that one cell has gotten to full chargeis used to reduce the charge current that individual cell receives.However, as will be pointed in more detail below, that information couldalso be used to reduce the charge current received by the entire seriesstack.

As another alternative, that information can be used to totallydiscontinue the charging of the entire series stack.

In the battery charger 10 shown in FIG. 2 the charge of each cell ismonitored. The charge current to that particular cell is controlled inresponse to the information obtained by the monitoring to preventovercharging of that cell.

The charge on the cell 19 is controlled operationally by the OpAmp U12and its resister divider working off of the voltage regulator V12. TheOpAmp U12 turns on the power transistor Q12 when the voltage across thecell 19 sets up to a certain selected point, all in the same way asdescribed in detail above with reference to the corresponding componentsfor the cell 17.

In like manner transistor Q1 operates to shunt current aside from cell13, and transistor Q2 operates to shunt current aside from cell 15 atthe proper points in the charge operation.

This cell voltage monitoring and shunting circuitry can be stacked upindefinitely for any number of lithium polymer battery cells.

Also, any number of matched secondary windings can be utilized.

It should be noted that the presently available electronics have limitedvoltage capability, in the sense that the best available OpAmps likethose used for U11, U12, etc., need more than 2.5 volts to work on. Thecharger 10 shown in FIG. 2 therefore includes an OpAmp U13 set up as anoscillator to do a charge pump operation through the capacitors C 14 andthe diode 11 and the diode 12 so that the voltage of V13 is usefullyhigher than any voltage in the cells 17 and 19 to provide operationalbias voltages for the electronics. If OpAmps could be obtained whichwould work down reliably at 2.5 volts, then the charge pump operationwould not be necessary. The charge pump components described immediatelyabove are auxiliary electronic equipment.

In the charger 10 shown in FIG. 2 two cells are connected in a seriesstack for receiving the power supply from a related secondary winding ofthe transformer.

As noted above, more than two cells could be connected in the seriesstack with each cell instrumented for monitoring the state of charge ineach cell.

The most practical number of cells in a stack will depend, to a certainextent, on the obtaining of statistical data regarding manufacturingtechniques and relative voltage characteristics for stacks of lithiumpolymer battery cells as this particular lithium polymer battery artdevelops further.

It should also be noted that only a single cell can be connected to arelated secondary winding, and, in that construction, the individualcell voltage monitoring electronics can be -eliminated. The secondarywindings can be sufficiently matched and the related rectifyingelectronics have sufficiently small tolerances so that the maximumoutput charging voltage of a secondary winding will equal the fullcharge voltage of the cell. Thus, in operation, a battery charger whichhas a 1 to 1 relationship between a secondary winding and a lithiumpolymer battery cell will bring the cell to full charge without anypossibility of overcharge.

The pulse width modulator 21 as described above operates to provide aconstant charge current until the monitored secondary reaches a finalvoltage. The particular circuitry shown in FIG. 2 uses a commerciallyavailable chip to pulse width modulate the signals to the powerresistors Q1 and Q2 which drive the transformer T1.

The circuit shown in FIG. 2 can have a control system whereby thevoltage control Vctl of the pulse width modulator chip 21 is used tocontrol the current. A pulse width modulation of the power transistorsQ1 and Q2 in that event would result in control of the voltage beingapplied to the cells and would thus control the current to whateveramount of current would be desired, rather than providing only aconstant charge current (as produced by the specific circuit shown inFIG. 2).

A controlled current power source embodiment of the present invention isshown in FIG. 3 and is indicated generally by the reference numeral 30in FIG. 3.

The charger 30 shown in FIG. 3 permits control of the charge current andpermits tapering the charge current down to some lower level at acertain point in the charge operation, rather than maintaining aconstant current output from the power supply 35 during the entirecharge operation.

Being able to reduce the charge current, particularly near the end ofthe charge cycle, minimizes the amount of power which is shunted aroundindividual cells and thereby minimizes the amount of heat which has tobe dissipated as a result of such shunting.

As illustrated in FIG. 3 the lithium power battery charger 30 comprisesa plurality of lithium polymer battery cells 13, 15, 17, and 19connected in a series stack and includes voltage survey means 31 formonitoring the state of charge of each cell in the stack. An outputdetector 33 determines the maximum voltage of any cell in the stack andtransmits a signal corresponding to the maximum voltage to thecontrolled current source 35 by a signal line 37.

When the maximum voltage of any cell in the stack approaches fullcharge, the signal on the line 37 controls the current source 35 toreduce the amount of current supplied to the series stack of cells.

The charge current may be reduced to a point where the charge current iscompletely cut off, or, preferably, the charge current is reduced to atrickle charge at a level which will not overcharge the cell having themaximum voltage but which will continue to charge the cells which havevoltages less than the full charge voltage.

The implementation of the control current source 35 can be a variabletransformer having a motor driven shaft with the motor driven inresponse to the signal transmitted on the line 37.

The control current source can also be a pulse width modulatedtransformer of the kind shown in FIG. 2 but having the pulse widthmodulation set up to control the current going into the transformerrather than a voltage.

Another embodiment of a battery charger for lithium polymer batterycells and constructed in accordance with the present invention is shownin FIG. 4 and is indicated generally by the reference numeral 40. Thecharger 40 provides selective charge control.

It should be noted that the charger 40 also allows for selectivedischarge control, during operational usage of the battery, to have allcells reach peak charge at the same time during subsequent charging.

The charger 40 shown in FIG. 4 includes voltage monitors having taps41-51 for monitoring the voltage of each cell in the stack.

The charger 40 also includes relays 53-61 for controlling the connectionof resistors 63-71 across the respective cells 13-20.

The resistors 63-71 serve as capacity adjustment loads.

The charger 40 thus comprises a charge balancing circuit. As all cellsbecome balanced, the cells rotate through the charge routine, as will bedescribed in more detail below.

The charger 40 shown in FIG. 4 employs the relay controlled resistor setassociated with the cells for shunting current around individual cellsat selected periods of time during the charging of the series stack oflithium polymer battery cells.

The charger 40 shown in FIG. 4 is particularly useful for charging aseries stack of cells when the charging history of each cell is known sothat the charge current to each cell can be varied during charging ofthe stack in dependence on the charging history (as well as in responseto detection that the state of charge of an individual cell isapproaching full charge).

In the operation of the battery charger 40 shown in FIG. 4, it is notnecessary to wait until a particular cell approaches full charge beforeshunting at least some of the charging current around that particularcell.

Instead, a fixed, arbitrary amount of charge current can be pulled awayfrom any particular cell at any particular time by engaging the relayfor that particular cell and by maintaining the relay engaged for thedesired amount of time.

By controlling the net charge to each cell during the chargingoperation, it is possible to make all cells reach peak voltage at thesame time and to therefore avoid the problem of dissipating the largeamounts of power which can be required to shunt current aroundsubstantially all of the cells during a time interval near the end ofthe charging operation.

To illustrate by way of an example, if it is known from the last cycleof charging which cell is going to get charged first, then, using adistinct resistor and relay, a current can be pulled away from that cellat any time.

Thus, if it is known that the charging operation will take about anhour, and if it is known that cell 13 is going to be the first cell toreach full charge, then, after about five minutes into the charge cycle,the relay 53 could be actuated to reduce cell 13's charge through thecapacity adjustment load resistor 63, for, say, fifteen minutes.

By way of continued example, if it is known that cell 19 gets to fullcharge just after cell 13, the relay 59 would be actuated at a selectedtime after the relay 53, to shunt some of the charge to cell 19 for,say, ten minutes.

The drops in the respective charge currents to the respective cells,resulting from selected actuations of the respective relays for selectedperiods of time, reduce the charges to the cells by selected amountsduring actuations of the respective relays.

The relay actuated shunts shown in the FIG. 4 embodiment are digitaldevices which shunt an arbitrary, fixed amount of current. This can havesome advantages over a shunt regulator (of the kind described earlierwith reference to FIG. 2) which is a linear device (a device in whichthe current through the shunt varies).

By proper control of the net charge to all cells, it is possible tocause all of the cells to reach peak voltage at the same time. Thisavoids the shunting of peak currents and peak power at the end of thecharging cycle. It avoids the need to dissipate the heat from suchshunted peak power at the end portion of the charging cycle.

A sample algorithm for determining the operation of the relays 53-61 isset out at the bottom part of FIG. 4 and is repeated immediately belowin this specification. Sample Algorithm:

CALCULATE: EOC+EOD FOR EACH CELL

2

FIND CELL WITH HIGHEST CALCULATED VALUE

IF HIGHEST CELL ALSO HAS LOWEST EOD, DO NOTHING

ELSE, APPLY LOAD TO HIGHEST CELL FOR A PREDETERMINED NUMBER OF MINUTESDURING NEXT CHARGE PERIOD

This algorithm, when incorporated in computer software, enables the mostdesired mode of operation of the relays 53-61 to be continuously refinedduring repeated cycles of recharging of the series stack of lithiumpolymer battery cells 13-20.

The charger 40 shown in FIG. 4 enables pre-emptive shunting of the cellsto be accomplished. By knowing which cell is going to get to full chargefirst, it is possible to shunt aside a bit of current to that cell at aconvenient time so that cell does not get to the end of charge beforethe desired time for reaching full charge.

Another embodiment of a charger for lithium polymer cells andconstructed in accordance with the present invention is shown in FIG. 5and is indicated generally by the reference numeral 50.

The charge 50 includes voltage monitors having taps 41-51 for monitoringthe voltage of each cell in the stack.

The charge 50 also includes single pole, double throw switches 53-57 forcontrolling the connections between the several cells 13-17 in thenominally series string. The term cells is to be interpreted literallyas a cell or figuratively as a group of cells or batteries in someseries/parallel combination, as might be found in electrical landvehicles or electrical by powered water vehicles.

The charger 50 (in the specific embodiment illustrated in FIG. 5) thusswitches cells completely out of the charge current path, substantiallywithout power dissipation, whenever desirable as indicated by themeasurements made from the voltage monitors. This removal from thecharge path may be done at end of charge or pre-emptively during thecharging process so that all cells reach full charge at substantiallythe same time.

Alternatively, the switching circuit may shunt a large fraction of thecharging current to such a cell or battery such that the cell or batteryis trickle charged at such a low rate as not to exceed the cutoffvoltage.

While we have illustrated and described the preferred embodiments of ourinvention, it is to be understood that these a re capable of variationand modification, and we therefore do not wish to be limited to theprecise details set forth, but desire to avail ourselves of such changesand alterations as fall within the purview of the following claims.

We claim:
 1. A lithium polymer battery charger apparatus for charging aplurality of equal charge point lithium polymer battery cells to fullycharged conditions without overcharging the cells, said apparatuscomprising,a plurality of equal charge point lithium polymer batterycells, each of the lithium polymer battery cells having a fully chargedcondition and being susceptible to serious damage in the event ofovercharging beyond said fully charged condition, a transformer havingone primary winding and a plurality of equally matched secondarywindings, each of the secondary windings connected to at least onelithium polymer battery cell, control means for controlling the voltageon the primary winding to produce a voltage in each secondary windingwhich is effective to charge each lithium power battery cell and tobring each lithium polymer battery cell to said fully charged conditionwithout overcharging, charge terminating means for discontinuing thecharging when all of the cells have been fully charged, and wherein onlyone lithium polymer battery cell is connected to a secondary winding andwherein the control means limit the maximum voltage on the primarywinding to that which will produce a voltage on each secondary windingcorresponding to the voltages needed to bring the lithium polymer cellsto said fully charged condition.
 2. A lithium polymer charger apparatusfor charging a plurality of equal charge point lithium polymer batterycells, said apparatus comprising,a plurality of equal charge pointlithium polymer battery cells, each of the lithium polymer battery cellshaving a fully charged condition, a transformer having one primarywinding and a plurality of equally matched secondary windings, each ofthe secondary windings connected to at least one lithium polymer batterycell, control means for controlling the voltage on the primary windingto produce a voltage in each secondary winding which is effective tocharge each lithium polymer battery cell and to bring each lithiumpolymer battery cell to said fully charged condition, charge terminatingmeans for discontinuing the charging when all of the cells have beenfully charged, and wherein multiple lithium polymer battery cells areconnected in series with a secondary winding and including detectingmeans for detecting when a particular lithium polymer battery cell isnearing full charge and wherein, in response to the detecting meansdetecting that a particular lithium polymer battery cell is nearing fullcharge, the control means shunt sufficient current around that cell toavoid overcharging that cell and continue the shunting until all of thecells have been fully charged.
 3. The apparatus defined in claim 2wherein the control means include transistor means for doing theshunting in an analog transition in which the amount of shunted currentis increased as the cell gets nearer to said fully charged condition. 4.The apparatus defined in claim 3 wherein the control means include relaymeans for doing the shunting in a digital transition in which a relay isenergized to shunt a fixed amount of current when the cell reaches thefully charged condition.